Abstract: The temperature-dependent characteristics of Formula heterojunction bipolar transistors with different emitter ledge thickness Formula are studied and demonstrated. From experimental results, devices Formula Formula and Formula Formula show the highest current gains, lowest base current, and base current ideality factors. Devices Formula and Formula also exhibit an improved thermal stability on dc current–voltage characteristics. Moreover, device Formula shows the best reliability performance after a Formula stress test. Therefore, the data support that the optimum emitter ledge thickness is between 100 and Formula .